White, biaxially oriented polyester films are known from the prior art.
DE-A 2 353 347 describes a process for producing a milky polyester film having one or more layers, which comprises preparing a mixture of particles of a linear polyester with from 3 to 27% by weight of a homopolymer or copolymer of ethylene or propylene, extruding the mixture as a film, quenching the film and biaxially orienting the same by stretching in directions running perpendicularly to one another, and heat-setting the film. A disadvantage of the process is that regenerated material produced during the production of the film (essentially a mixture of polyester and ethylene or propylene copolymer) cannot be reused without discoloration or yellowing of the film. This makes the process uneconomic. In addition, the film has high roughness values, giving it a very matt appearance (very low gloss), and this is undesirable for many applications.
EP-A 0 300 060 describes a single-layer polyester film which comprises, besides polyethylene terephthalate, from 3 to 40% by weight of a crystalline propylene polymer and from 0.001 to 3% by weight of a surface-active substance. The effect of the surface-active substance is to increase the number of vacuoles in the film and at the same time to reduce their size to the extent desired. This achieves higher opacity and lower density of the film. A disadvantage of the film continues to be that regenerated material produced during the production of the film (essentially a mixture of polyester and propylene homopolymer) cannot be reused without yellowing of the film. However, this makes the process uneconomic. In addition, the film has high roughness values, giving it a very matt appearance (very low gloss), and this is undesirable for many applications.
EP-A 0 360 201 describes a polyester film having at least two layers, comprising a base layer with fine vacuoles, the density of which is from 0.4 to 1.3 kg/dm3, and at least one outer layer, the density of which is greater than 1.3 kg/dm3. The vacuoles are achieved by adding from 4 to 30% by weight of a crystalline propylene polymer, followed by biaxial stretching of the film. As a result of adding the outer layer the ease of production of the film is improved (no streaking on the film surface), the surface tension is increased and the roughness of the laminated surface can be reduced. A disadvantage still present is that regenerated material produced during the production of the film (essentially a mixture of polyester and propylene homopolymer) cannot be reused without yellowing of the film. This makes the process uneconomic. In addition, the films listed in the examples have high roughness values, giving them a matt appearance (low gloss), and this is undesirable for many applications.
EP-A 0 795 399 describes a polyester film having at least two layers and comprising a base layer with fine vacuoles, the density of which is from 0.4 to 1.3 kg/dm3, and having at least one outer layer, the density of which is greater than 1.3 kg/dm3. The vacuoles are produced by adding from 5 to 45% by weight of a thermoplastic polymer to the polyester in the base layer, followed by biaxial stretching of the film. The thermoplastic polymers used are, inter alia, polypropylene, polyethylene, polymethylpentene, polystyrene or polycarbonate, and the preferred thermoplastic polymer is polypropylene. As a result of adding the outer layer the ease of production of the film is improved (no streaking on the film surface), the surface tension is increased and the roughness of the laminated surface can be matched to prevailing requirements. Further modification of the film in the base layer and/or in the outer layers, using white pigments (generally TiO2) and/or using optical brighteners, permits the properties of the film to be matched to the prevailing requirements of the application. A continuing disadvantage is that regenerated material produced during the production of the film (essentially a mixture of polyester and the added polymer) cannot be reused without undefined changes in the color of the film, this being undesirable in many applications. This makes the process uneconomic. In addition, the films listed in the examples have high roughness values, giving them a matt appearance (low gloss), and this is undesirable for many applications.
DE-A 195 40 277 describes a polyester film having one or more layers, which comprises a base layer with fine vacuoles, the density of which is from 0.6 to 1.3 kg/dm3, and which has a planar birefringence of from −0.02 to 0.04. The vacuoles are produced by adding from 3 to 40% by weight of a thermoplastic resin to the polyester in the base layer, followed by biaxial stretching of the film. The thermoplastic resins used are, inter alia, polypropylene, polyethylene, polymethylpentene, cyclic olefin polymers, polyacrylic resins, polystyrene or polycarbonate, and preferred polymers are polypropylene and polystyrene. Maintaining the stated limits for the birefringence of the film gives the claimed film in particular a superior tear strength and superior isotropic properties. A continuing disadvantage is that regenerated material produced during the production of the film cannot be reused without undefined changes in the color of the film, this being undesirable for many applications. However, this makes the process uneconomic. In addition, the films listed in the examples have high roughness values, giving them a matt appearance (low gloss), and this is undesirable for many applications.
It was therefore the object of the present invention to provide a white polyester film which in particular has very high gloss and improved ease of production, i.e. low production costs, but without the disadvantageous properties mentioned above. In particular, it should be possible for regrind arising directly during the production process to be reused, preferably at a concentration of from 10 to 70% by weight, based on the total weight of the film, without any significant resulant adverse effect on the physical properties of the film. In particular, the addition of regrind should not cause discoloration or yellowing of the film.